1. Field of the Invention
This invention relates to an amplifier circuit for amplifying an input signal and transmitting the amplified signal as an output signal, comprising:                a first supply terminal and a second supply terminal for supplying the circuit with a first supply potential and a second supply potential respectively,        at least one current path which runs from the first supply terminal via a first biased transistor, a first circuit node, an input transistor, a second circuit node and a second biased transistor to the second supply terminal, wherein a control terminal of the input transistor is loaded with the input signal, and wherein the second circuit node forms a pick-up in a resistor chain at whose ends the output signal is supplied as a voltage drop, and        a feedback stage for enabling the current to flow through the resistor chain dependent on the input signal so that the current flowing through the input transistor is essentially independent of the input signal, where the feedback stage has a pair of transistors coupled for complementary operation with an intermediate output node for transmitting the current.        
2. Description of the Prior Art
Such an amplifier circuit is disclosed, for example, in WO 2005/029711 A2. FIG. 7 of this publication, for example, shows an amplifier circuit constructed symmetrically of two amplifier branches for amplifying a differential input signal (input potentials INP, INL) and transmitting the amplified signal as a differential output signal (output potentials OPH, OPL). The amplifier circuit is supplied by means of a first supply potential Vpos and a second supply potential COM.
The amplifier branch shown in the left-hand part of the figure comprises a current path running from the first supply potential VPOS via an input transistor Q15 to the second supply potential COM, wherein a control terminal of the input transistor is loaded with one of the input potentials and wherein the emitter of the input transistor is connected to a pick-up in a resistor chain R2, R1, R3 to whose ends the output signal is supplied as a voltage drop.
The circuit part shown in the figure to the left of this current path forms a feedback stage enabling the current to flow through the resistor chain dependent on the input signal so that the current flowing through input transistor Q15 is essentially independent of the input signal. A certain working point of the input transistor is therefore advantageously retained in the operation of the amplifier. The current required to enable it to flow through the resistor chain is supplied to a pair of transistors Q6, Q7 coupled for complementary operation with an intervening output node connected to the resistor chain.